The positioning and deployment of medical devices within a target site of a patient is a common, often-repeated procedure of contemporary medicine. These devices or implants are used for innumerable medical purposes including the reinforcement of recently re-enlarged lumens and the replacement of ruptured vessels.
Coatings are often applied to the surfaces of these medical devices to increase their effectiveness. These coatings may provide a number of benefits including reducing the trauma suffered during the insertion procedure, facilitating the acceptance of the medical device into the target site, and improving the post-procedure effectiveness of the device.
Coating medical devices also provides for the localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Such localized drug delivery avoids the problems of systemic drug administration, such as producing unwanted effects on parts of the body which are not to be treated, or not being able to deliver a high enough concentration of therapeutic agent to the afflicted part of the body. Localized drug delivery is achieved, for example, by coating expandable stents, grafts, or balloon catheters, which directly contact the inner vessel wall, with the therapeutic agent to be locally delivered. Expandable stents are tube-like medical devices that often have a mesh-like patterned structure designed to support the inner walls of a lumen. These stents are typically positioned within a lumen and, then, expanded to provide internal support for it. The coating on these medical devices may provide for controlled release, which includes long-term or sustained release, of a bioactive material.
Aside from facilitating localized drug delivery, medical devices are coated with materials to provide beneficial surface properties. For example, medical devices are often coated with radiopaque materials to allow for fluoroscopic visualization during placement in the body. It is also useful to coat certain devices to achieve enhanced biocompatibility and to improve surface properties such as lubriciousness.
Conventionally, coatings have been applied to medical devices by processes such as dipping and spraying. These coating processes are, however, indiscriminate, wasteful, and difficult to control. For example, because dip-coating or spray-coating processes often indiscriminately coat the internal surface of a patterned medical device as well as the external surface, expensive coating materials, such as therapeutic agents, are wasted. In addition, when a coated medical device, such as a coated stent, is placed within a blood vessel, the wasted therapeutic coating on the internal surface of the stent washes directly into the bloodstream instead of treating the diseased walls of the lumen. Due to the toxic nature of some therapeutics, the loss of therapeutic agents into the blood stream should be minimized.
Conventional coating processes, such as dipping and spraying, also cannot apply multiple layers of different coatings without requiring appropriate drying time between coating steps. This increases production time and costs. Also, it is often difficult to achieve coatings of uniform thicknesses, thereby placing more coating at one particular region of the medical device, making it difficult to predict the dosage of therapeutic that will be delivered. Such coating defects can compromise the stent's effectiveness.
Conventional coating techniques have drawbacks in the application of thick coating layers. Because thick coatings using a dip coating method require multiple dipping steps, and the dip coating solvent often dissolves a portion of the underlying dip coating upon a second dipping step, it is difficult to control the application of thick coatings. Spray coatings require multiple coating steps to achieve a desired coating thickness, and are thus inefficient. Also, conventional spray coating processes are limited to low viscosity coating solutions.
There is, therefore, a need for a cost-effective method and apparatus for coating the surface of medical devices that results in uniform coatings and uniform drug doses per unit device. The method would minimize coating of the internal surfaces of the medical device and also allow for the simultaneous application of multiple coating layers of high and low viscosity.